Table 2.

Intraclass Correlations for Same- and Different-Class Twins Pairs (N Pairs in Parentheses; Contrasts Significant at .01 in Bold)

Grade	Test	MZ pairs						DZ pairs
		US		Australia		Total		US		Australia		Total
		S	D	S	D	S	D	S	D	S	D	S	D
Kindergarten		(171, 53)		(85, 46)		(256, 99)		(174, 85)		(66, 31)		(240, 116)
	TOWRE SWE	.89	.84	.85	.82	.88	.83	.64	.51	.51	.41	.61	.50
	TOWRE PDE	.76	.61	.73	.54	.75	.59	.55	.53	.38	.34	.51	.52
	Spelling	.78	.75	.83	.73	.80	.75	.59	.52	.46	.55	.56	.55
	Total literacy	.88	.79	.90	.76	.89	.78	.59	.57	.54	.42	.58	.54
Grade 1		(95, 124)		(58, 66)		(153, 190)		(97, 156)		(45, 46)		(142, 202)
	TOWRE SWE	.88	.86	.84	.76	.86	.83	.49	.48	.36	.32	.46	.45
	TOWRE PDE	.83	.73	.83	.80	.81	.76	.34	.33	.44	.44	.41	.41
	WRAT Spelling	.82	.73	.84	.60	.82	.69	.48	.41	.23	.33	.41	.39
	WPC	.74	.82	.82	.66	.77	.77	.58	.40	.51	.29	.56	.37
	Total literacy	.88	.86	.90	.74	.89	.82	.55	.46	.36	.34	.50	.44
Grade 2		(73, 105)		(43, 68)		(116, 173)		(55, 158)		(34, 40)		(89, 198)
	TOWRE SWE	.88	.79	.79	.78	.84	.79	.55	.41	.46	.24	.52	.38
	TOWRE PDE	.85	.75	.80	.83	.83	.78	.50	.46	.35	.20	.45	.42
	WRAT Spelling	.84	.77	.78	.77	.81	.76	.54	.35	.35	−.05	.48	.29
	WPC	.72	.70	.76	.66	.73	.68	.56	.38	.52	.16	.54	.33
	Total literacy	.90	.84	.84	.76	.88	.81	.60	.47	.40	.24	.55	.41

Note: S = Same class, D = Different class; TOWRE SWE = Test of Word Recognition Efficiency, Sight Word Efficiency; TOWRE PDE = Test of Word Recognition Efficiency, Phonemic Decoding Efficiency; WRAT Spelling = Wide Range Achievement Test, Spelling; WPC = Woodcock Reading Mastery Test (Revised), Passage Comprehension. Total literacy in each grade is based on mean of all tests at that grade for each child.